The present invention relates to new and improved materials for thermoelectric applications and devices made therewith.
It has been recognized that the world supply of fossil fuels for the production of energy is being exhausted at ever increasing rates. This realization has resulted in an energy crisis which impacts not only the world's economy, but threatens the peace and stability of the world. The solution to the energy crisis lies in the development of new fuels and in more efficient techniques for energy conversion which avoid the dependence on fossil fuels.
An important part of the solution with respect to the development of permanent, economical energy conversion lies in the field of thermoelectrics wherein electrical power is generated by heat and where, via the Peltier effect, a temperature gradient may be generated from electric power to accomplish heating or cooling.
The performance of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the material forming the device, wherein Z is defined as: ##EQU1## where: Z is expressed in units.times.10.sup.3 /.degree.K.
S is the Seebeck coefficient in .mu.V/.degree.C. PA1 K is the thermal conductivity in mW/cm-.degree.K. PA1 .sigma. is the electrical conductivity in (.OMEGA.cm).sup.-1
From the above, one can see that in order for a material to have a high figure of merit, it must have a relatively large value for the Seebeck coefficient (S), a high electrical conductivity (.sigma.), and a low thermal conductivity (K). Further, there are two components to the thermal conductivity (K): K.sub.l, the lattice component; and K.sub.e, the electrical component. In non-metals and semiconductors, K.sub.l dominates and it is this component which mainly determines the value of K.
Stated in another way, in order for a material to be efficient for thermoelectric conversion, it is important to allow charge carriers to diffuse easily from the hot junction to the cold junction while maintaining the temperature gradient. Hence, high electrical conductivity is required along with low thermal conductivity.
Thermoelectric conversion has not found wide usage in the past. The major reason for this is that prior art thermoelectric materials which have been proposed for commercial applications have been primarily crystalline in structure. Those crystalline materials best suited for thermoelectric devices are very difficult to manufacture because of poor mechanical properties and sensitivity of material porperties to compositional changes. This results because they contain a predominance of elements, such as tellurium, and selenium, which are natural glass formers. The growth, control, and mechanical stability of these crystals have, therefore, led to what to this date are insurmountable problems. In particular, materials having a high figure of merit are generally made from chalcogenides such as tellurium compounds which are notorious for the difficulties in growth of high-quality single crystals. Even when such crystals are grown, they contain large densities of defects and are often unstable. In addition, they usually are far from stoichiometric. For all of these reasons, controlled doping has proven to be extremely difficult.
Crystalline solids cannot attain large values of electrical conductivity while maintaining low thermal conductivity. Most importantly, because of crystalline symmetry, thermal conductivity cannot be independently controlled by modification.
In the case of the polycrystalline approach, the problems of single crystalline materials still dominate. However, new problems are also encountered by virtue of the polycrystalline grain boundaries which cause these materials to have relatively low electrical conductivities. In addition, the fabrication of these materials is also difficult to control as a result of their more complex crystalline structure. The chemical modification or doping of these materials, because of the above problems is especially difficult.
Among the best known currently existing polycrystalline thermoelectric materials are those based on Bi.sub.2 Te.sub.3, Pb-Te, and Si-Ge. The Si-Ge materials are best suited for high temperature applications in the 600.degree. to 1000.degree. C. range with a satisfactory Z appearing at above 700.degree. C. The PbTe polycrystalline material exhibits its best figure of merit in the 300.degree. to 500.degree. C. range. Neither of these materials is well suited for applications below 300.degree. C. This is indeed unfortunate, because it is in this temperature range where a wide variety of applications are found.
Extensive efforts have been made to improve the figure-of-merit for bismuth telluride systems for temperatures ranging from 300.degree. C. and downward. Variations have been suggested in materials (e.g. alloying components and dopants), morphology (crystalline or disordered (amorphous, microcrystalline, polycrystalline)) production technique (crystal growth, quenching, powder pressing, annealing, sintering, etc.), and in other variables.
Disordered materials are ideally suited for manipulation since they are not constrained by the symmetry of a single phase crystalline lattice or by stoichiometry. By moving away from materials having such restrictive single phase crystalline symmetry, it is possible to accomplish a significant alteration of the local structural and chemical environments involved in thermoelectric conversion efficiency.
The types of disordered structures which provide the local structural chemical environments for the enhanced efficiency contemplated by the invention include many types. The following list provides a classification of the spectrum of disordered structures contemplated by the present invention:
1. Multicomponent polycrystalline materials lacking long-range compositional order.
2. Microcrystalline materials.
3. Mixtures of polycrystalline and microcrystalline phases.
4. Mixtures of polycrystalline or microcrystalline and amorphous phases.
5. Amorphous materials containing one or more amorphous phases.
Disordered thermoelectric materials are fully disclosed in copending U.S. application Ser. No. 412,306, filed Aug. 25, 1982 now U.S. Pat. No. 4,447,277 in the names of Tumkur S. Jayadev and On Van Nguyen for "New Multiphase Thermoelectric Alloys and Method of Making Same", and in copending U.S. Ser. No. 414,917 filed Sept. 3, 1982 by Jayadev et. al. entitled "New Powder Pressed Thermoelectric Materials And Method of Making Same." both of which are incorporated herein by reference. Materials of this type are good thermal insulators.
Amorphous materials, representing the highest degree of disorder, have been made for thermoelectric applications. The materials and methods for making the same are fully disclosed and claimed, for example, in U.S. Pat. Nos. 4,177,473, 4,177,474, and 4,178,415 which issued in the name of Stanford R. Ovshinsky. The materials disclosed in these patents are formed in a solid amorphous host matrix having structural configurations which have local rather than long range order and electronic configurations which have an energy gap and an electrical activation energy. Added to the amorphous host matrix is a modifier material having orbitals which interact with the amorphous host matrix as well as themselves to form electronic states in the energy gap.
Alloy and dopant materials have been suggested for use with various thermoelectric materials but generally those adjuvants useful for one system cannot be used with the same beneficial results in systems of different composition or morphology.
In "Materials for Thermoelectric Refrigeration" J. Physical Chemistry V.10 Pp. 191-200 (1959) adjuvants are suggested for crystalline p- and n-type materials. Suggested for p-type are all elements in Groups IV and V such as Sn, Pb, Sb, As and excess Bi. For n-type, Cu.sub.2 S and the iodides and bromides of copper and silver are suggested for the optimum system containing 15 to 25% Bi.sub.2 Se.sub.3 in crystalline form. Other publications suggest additions for n-types including tellurium iodide, silver/antimony/tellurium, cuprous bromide, sulfur and elemental copper.
Alloys or dopants effective in a given system, aren't necessarily useful in other systems of differing components and/or morphology. For example, tellurium iodide, silver/antimony/tellurium, cuprous bromide, sulfur and elemental copper have proven ineffective in the present invention through apparently useful with prior materials.
The use of pressed powder materials as thermoelectric elements has been suggested, for example, in copending U.S. application Ser. No. 414,917 filed Sept. 3, 1982 in the names of Jayadev et. al. previously mentioned wherein a multiphase material based on bismuth and tellurium including a highly conductive second phase is pressed to form a thermoelectric element.
The materials of the present invention are also based on bismuth and tellurium and are powder pressed to form thermoelectric elements. The materials are more simply prepared than in the above process and incorporate alloy concentrations and/or dopants to form improved thermoelements. The thermoelements of the invention exhibit improved properties for thermoelectric applications, particularly at temperatures of 300.degree. C. and below and especially for cooling. This is the range of operation of most heating and cooling devices and many electrical generation applications.